1. Field of the Invention
The invention relates to plunger-type or barrel locks and keys.
2. Description of the Prior Art
Locks having internal spring-loaded axially movable plungers and radially extending locking members such as steel balls, are widely used to prevent tampering with gas and electric meters. Examples of such locks, called generally plunger-type locks throughout this specification are shown in U.S. Pat. Nos. 1,923,025 and 3,714,802 to Morse et al; 3,002,368 and 3,033,016 to Moberg; and 3,835,674 to Hoyt. The locks of all of the foregoing patents are intended to prevent unlocking by any means other than the proper key. The ingenuity and resourcefulness of those who seek unauthorized access to electric and gas meters and shutoff switches and valves, learned through sad experience by utility companies, have been countered by advances in the technology of locks of the plunger type.
Morse et al U.S. Pat. No. 1,923,025 shows a plunger-type lock and key in which threads on the key mate with internal threads of a spring-loaded plunger for retraction of the plunger.
The lock of Moberg U.S. Pat. No. 3,002,368 was designed to be opened with a key shown in Moberg U.S. Pat. No. 3,033,016, which has a pair of fingers that are spread apart within a bore of the lock's plunger to engage frictionally and permit retraction of the plunger.
Morse et al U.S. Pat. No. 3,714,802, noting that the locks of the Morse et al and Moberg patents discussed above have been attacked with occasional success by the use of a wire with a return bend at its end, proposed the insertion of a relatively free and slidable sleeve within the axial bore of the lock plunger to thwart bent-wire lock-pickers by requiring a greater degree of radially expanding force. This patent also suggests limiting the depth of the bore in the lock's plunger.
Hoyt U.S. Pat. No. 3,835,674 is directed to defense against the lock picker who hammers a nail into the plunger bore of the previously described locks and thus attains sufficient engagement to retract the plunger and free the locking balls to open the lock. Hoyt's plunger is made in two parts, one of which must be retracted while the other remains in place to open the lock so that the nail-hammering lock-picker would wedge both parts together and retract both parts in a vain attempt to tamper with a meter. The piece which is to be left behind in opening Hoyt's lock has a conical point facing the keyhole. However, the limited cavity provided by the plunger bore of the Hoyt lock makes gripping the plunger, even with the authorized key difficult, especially is someone has damaged the lock by attempting to pick it. If the conical point on the central member of Hoyt's lock has been bent over to engage the cylinder by a hammering lock-picker, even the authorized key cannot open the lock. Foreign matter can lodge in the bore of Hoyt's lock, hampering operation and preventing opening of the lock with its proper key, so that when an authorized person removes the lock, he may be required to destroy parts held by the lock.
It has also been found that some locks using keys of the internal frictional engagement type can be opened by means of a device of the type shown in U.S. Pat. No. 1,656,180, and though the casual tamperer probably does not have such a tool, it is available and may be used by the more experienced practitioner of the lock-picking art.
The prior art locks will succumb to brute force of sufficient violence or to some combination of skill and luck on the part of the lock-picker, but entirely aside from these problems is the unauthorized possession and use of the key designed for the lock. The loss or theft of a key exposes to unauthorized entry the many locks for which the key is intended, and despite vigilance in restricting access to keys, there have been cases in which keys have been unlawfully used.
The security problem of lost and stolen keys has been difficult to combat because not only those who must open the lock but also those who install locks of the prior art types discussed by way of example, ordinarily have authorized keys for the locks. Because personnel are required to have keys for the prior art locks, control, record keeping and administration related to the keys has been difficult.
The foregoing drawbacks of conventional plunger-type lock and key systems evidence the present need for a pick resistant plunger-type lock that cannot be opened by existing keys, but which can be opened by means of a key which will also open existing locks, facilitating a "master key" concept.